Optical devices formed from doped silica planar waveguides are typically fabricated by depositing silica layers having differing dopants on a substrate. A significant problem with these devices is that strain birefringence arises because the various layers have different thermal expansion coefficients that cause large strains once the device returns to ambient temperature after undergoing annealing during the fabrication process. Birefringence is defined in terms of the principle indices of refraction n.sub.TE and n.sub.TM respectively experienced by the TE and TM modes of an optical signal. More specifically, the birefringence is equal to the difference between the refractive indices and thus is zero when the two refractive indices are equal. While numerous factors contribute to birefringence, strain induced birefringence is believed to be by far the largest component in planar silica devices.
Optical devices are often required to perform in a manner insensitive to the polarization state of the input optical signal. Strain induced birefringence in silica waveguide devices can seriously degrade performance in this respect. For example, the difference between the two principle refractive indices in silica waveguides which is attributable to strain is typically measured to be about 3.times.10.sup.-4. An optical signal propagating in a device such as a multiplexer/demultiplexer which incorporates silica waveguides experiences a wavelength dependence on polarization of about 0.3 nm, which is unacceptably large for many applications. In telecommunications wavelength-division multiplexed systems for example, a maximum polarization wavelength dependence of about 0.05 nm is required.
A considerable amount of work has been done on radiolytically induced stress, strain and birefringence in silicate materials, which are known to be photosensitive. In this context a material is described as being photosensitive if its refractive index changes upon irradiation. For example, as demonstrated by Rothschild et al., Appl. Phys. Lett., vol. 55, pp.1276-1278, 1989, it is well known that birefringence may be induced in an isotropic bulk material that initially exhibits no birefringence. Specifically, birefringence can be increased from its initial value of zero by irradiating a limited region of the material. Resulting stress between the irradiated and non-irradiated regions causes birefringence.
Other studies performed on planar waveguides such as reported in Hibino et al., Electon. Lett., vol. 29, pp. 621-623, 1993, indicate that birefringence can be reduced by irradiating the waveguide structure at a wavelength which is absorbed by photosensitive defects in the core material. In particular, Hibino demonstrated that birefringence can be reduced in germanium-doped planar silica waveguides by irradiation with ultraviolet light at a wavelength of 248 nm. This wavelength corresponds to germania-related color center defects that are known to be present in the silica core material. In Hibino, the photosensitivity of the germanium core was enhanced by consolidating the core material in a reducing atmosphere. This technique is well known to facilitate the generation of germanium defects responsible for optical absorption at. 248 nm. Since the cladding composition is presumably a standard phosphorous and boron-doped silica layer, the cladding is essentially transparent to light at 248 nm, with an optical absorption coefficient on the order of 0.1 cm.sup.-1. By comparison, the optical absorption coefficient of the germanium-doped core layer is on the order of 100 cm.sup.-1 at this wavelength. FIG. 3 of Hibino indicates that irradiation induces a more rapid change in n.sub.TE than in n.sub.TM. However, the data suggests to those skilled in the art that the photoinduced refractive index changes saturate before n.sub.TM is substantially equal to n.sub.TE, that is, before birefringence is substantially eliminated.
Similar to Hibino, Wong et al., Opt. Lett., 1992, 17, pp. 1773-1775, induced refractive index changes in germanium-doped silica fibers with radiation at 248 nm. More particularly, Wong measured the temperature sensitivity of the birefringence before and after irradiation. Wong demonstrated that the temperature dependence of the birefringence decreases after exposure. Wong proposed a model to explain this phenomenon which implies that birefringence cannot be entirely eliminated. Their model also implies that their process cannot cause the birefringence to change sign.
The photosensitivity of silica materials is sometimes employed to alter the refractive index without affecting the birefringence. For example, Kitagawa et al., Electon. Lett., 1994, 30, p 1311, forms optical gratings by photoinducing through a mask spatially nonuniform refractive index changes in phosphorous-doped silica waveguides. The photoinduced refractive index changes are performed at a wavelength of 193 nm. The waveguides are first loaded with molecular hydrogen to increase their photosensitivity. Kitagawa states that the magnitude of the resulting grating reflectivity was identical for both the TE and TM modes. Since the photoinduced index modulation determines the magnitude of the grating reflectivity, their result implies that the refractive index changes for both the TE and TM modes were identical. Kitagawa thus utilizes isotropic refractive index changes (i.e., index changes that are the same for both the TE and TM modes) to modify the refractive index of distinct waveguide segments so that the resulting device is configured as an optical grating. Consistent with their observations concerning the reflectivity, Kitagawa does not indicate that any anisotropic index changes (i.e., birefringence changes) occur in the waveguides.
While the prior art has reduced birefringence in a single waveguide structure with radiation having a wavelength that is relatively strongly absorbed by the waveguide core, there is no method for selecting an appropriate wavelength that reduces or even eliminates birefringence in a wide variety of waveguides having different core and cladding compositions.